The risk of recurrence for Down's syndrome in a pregnancy following one previously affected can be estimated based on the age of the woman and the genetic abnormality, as described by Donnai (1988). The risk of recurrence is low if the mother is young and if the affected gestation showed trisomy 21.
The essential step is to determine the chromosomal basis for the Down's syndrome in the affected infant:
(1) trisomy 21 (over 95% of affected infants)
(2) translocation, with the additional chromosome 21 attached to chromosome 21 (rare)
(3) translocation, with the additional chromosome 21 attached another chromosome, typically chromosome 14.
where:
• In a translocation there is an additional chromosome 21 but it is attached to another chromosome.
• Either parent may be a translocation carrier, so both parents need to be tested if a translocation is identified.
If the case was trisomy 21, then the risk varies with the maternal age.
(1) If the woman is 35 years of age or younger, then the risk ranges from 0.5% (Harper) to 1-2% (Donnai). The risk of all chromosomal abnormalities is about 1%.
(2) For a woman under 25 years of age, this is significantly higher than the risk based on age alone.
(3) For a woman older than 35, the recurrence rate approaches that determined by the woman's age.
|
Maternal Age
|
Risk
|
|
35
|
0.39%
|
|
36
|
0.5%
|
|
37
|
0.64%
|
|
38
|
0.81%
|
|
39
|
1.04%
|
|
40
|
1.33%
|
|
41
|
1.69%
|
|
42
|
2.16%
|
|
43
|
2.74%
|
|
44
|
3.48%
|
|
45
|
4.42%
|
|
46
|
5.59%
|
|
47
|
7.04%
|
from amniocentesis data, Ferguson-Smith. Br Med Bull. 1983; 355-364.
If the case is a translocation, the risk depends on the type of translocation and the parent who is the carrier.
|
Rate of Affected Offspring
|
Mother
|
Father
|
|
translocation, to chromosome 21
|
100%
|
100%
|
|
translocation, to another chromosome
|
10-15%
|
2-3%
|
